1. Field of the Invention
The present invention relates generally to an apparatus and method for providing transmit antenna diversity in a wireless communication system, and in particular, to an apparatus and method for space-frequency block coding (SFBC) to achieve a full diversity gain and a full rate in a mobile communication system using multiple antennas.
2. Description of the Related Art
The basic issue in communications is how to efficiently and reliably transmit data on channels. In addition to satisfying the demand for a high-speed communication system capable of processing and transmitting video and wireless data in addition to the traditional voice service, future-generation multimedia mobile communication systems, now under active study, increase system efficiency using an appropriate channel coding scheme.
Generally, in the wireless channel environment of a mobile communication system, unlike that of a wired channel environment, a transmission signal inevitably experiences loss due to several factors such as multipath interference, shadowing, wave attenuation, time-varying noise, and fading.
The resulting information loss causes a severe distortion to the actual transmission signal, degrading the whole system performance. In order to reduce the information loss, many error control techniques are usually adopted depending on the characteristics of channels to thereby increase system reliability. One basic technique is to use an error correction code.
Multipath fading is relieved by diversity techniques in the wireless communication system. The diversity techniques are classified into time diversity, frequency diversity, and antenna diversity. Antenna diversity uses multiple antennas. This diversity scheme is further branched into receive (Rx) antenna diversity using a plurality of Rx antennas, transmit (Tx) antenna diversity using a plurality of Tx antennas, and multiple-input multiple-output (MIMO) using a plurality of Tx antennas and a plurality of Rx antennas. MIMO is a special case of space-time coding (STC) that extends coding in the time domain to the space domain by transmission of a signal encoded in a predetermined coding method through a plurality of Tx antennas, with the aim to achieve a lower error rate.
V. Tarokh, et al. proposed STBC as one of methods of efficiently applying the antenna diversity scheme (see “Space-Time Block Coding from Orthogonal Designs”, IEEE Trans. On Info., Theory, Vol. 45, pp. 1456-1467, July 1999). The Tarokh STBC scheme is an extension of the transmit antenna diversity scheme of S. M. Alamouti (see, “A Simple Transmit Diversity Technique for Wireless Communications”, IEEE Journal on Selected Area in Communications, Vol. 16, pp. 1451-1458, October 1988), for two or more Tx antennas.
FIG. 1 is a block diagram of a transmitter in a mobile communication system using a conventional STBC. As proposed by Tarokh, the transmitter is comprised of a modulator 100, a serial-to-parallel (S/P) converter 102, an STBC coder 104, and four Tx antennas 106, 108, 110 and 112. Referring to FIG. 1, the modulator 100 modulates input information data (or coded data) in a predetermined modulation scheme. The modulation scheme can be one of binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), quadrature amplitude modulation (QAM), pulse amplitude modulation (PAM), and phase shift keying (PSK).
The S/P converter 102 parallelizes serial modulation symbols received from the modulator 100, s1, s2, s3, s4. The STBC coder 104 creates eight symbol combinations by STBC-encoding the four modulation symbols, s1, s2, s3, s4 and sequentially transmits them through the four Tx antennas 106 to 112. A coding matrix used to generate the eight symbol combinations is expressed as Equation (1):
                              G          4                =                  [                                                                      s                  1                                                                              s                  2                                                                              s                  3                                                                              s                  4                                                                                                      -                                      s                    2                                                                                                s                  1                                                                              -                                      s                    4                                                                                                s                  3                                                                                                      -                                      s                    3                                                                                                s                  4                                                                              s                  1                                                                              -                                      s                    2                                                                                                                        -                                      s                    4                                                                                                -                                      s                    3                                                                                                s                  2                                                                              s                  1                                                                                                      s                  1                  *                                                                              s                  2                  *                                                                              s                  3                  *                                                                              s                  4                  *                                                                                                      -                                      s                    2                    *                                                                                                s                  1                  *                                                                              -                                      s                    4                    *                                                                                                s                  3                  *                                                                                                      -                                      s                    3                    *                                                                                                s                  4                  *                                                                              s                  1                  *                                                                              -                                      s                    2                    *                                                                                                                        -                                      s                    4                    *                                                                                                -                                      s                    3                    *                                                                                                s                  2                  *                                                                              s                  1                  *                                                              ]                                    (        1        )            where G4 denotes the coding matrix for symbols transmitted through the four Tx antennas 106 to 112 and s1, s2, s3, s4 denote the input four symbols to be transmitted. The number of the columns of the coding matrix is equal to the number of the Tx antennas and the number of the rows corresponds to the time intervals required to transmit the four symbols. Thus, the four symbols are transmitted through the four Tx antennas for eight time intervals. Specifically, for a first time interval, s1 is transmitted through the first Tx antenna 106, s2 through the second Tx antenna 108, s3 through the third Tx antenna 110, and s4 through the fourth Tx antenna 112. Similarly, −s*4, −s*3, s*2, −s*1 are transmitted through the first to fourth Tx antennas 106 to 112, respectively for an eighth time interval. That is, the STBC coder 104 sequentially provides the symbols of an ith column in the coding matrix to an ith Tx antenna.
As described above, the STBC coder 104 generates the eight symbol sequences using the four input symbols and their conjugates and negatives and transmits them through the four Tx antennas 106 to 112 for eight time intervals. Since the symbol sequences for the respective Tx antennas, that is, the columns of the coding matrix are mutually orthogonal, a diversity gain as high as a diversity order is achieved.
FIG. 2 is a block diagram of a receiver in the mobile communication system using the conventional STBC scheme. The receiver is the counterpart of the transmitter illustrated in FIG. 1. The receiver is comprised of a plurality of Rx antennas 200 to 202, a channel estimator 204, a signal combiner 206, a detector 208, a parallel-to-serial (P/S) converter 210, and a demodulator 212. The first to Pth Rx antennas 200 to 202 provide signals received from the four Tx antennas of the transmitter illustrated in FIG. 1 to the channel estimator 204 and the signal combiner 206. The channel estimator 204 estimates channel coefficients representing channel gains from the Tx antennas 106 to 112 to the Rx antennas 200 to 202 using the signals received from the first to Pth Rx antennas 200 to 202. The signal combiner 206 combines the signals received from the first to Pth Rx antennas 200 to 202 with the channel coefficients in a predetermined method. The detector 208 generates hypothesis symbols by multiplying the combined symbols by the channel coefficients, calculates decision statistics for all possible transmitted symbols from the transmitter using the hypothesis symbols, and detects the actual transmitted symbols through threshold detection. The P/S converter 210 serializes the parallel symbols received from the detector 208. The demodulator 212 demodulates the serial symbol sequence in a predetermined demodulation method, thereby recovering the original information bits.
As stated earlier, the Alamouti STBC technique offers the benefit of achieving as high a diversity order as the number of Tx antennas, namely a full diversity order, without sacrificing data rate by transmitting complex symbols through two Tx antennas only. By comparison, the Tarokh STBC scheme extended from the Alamouti STBC scheme achieves a full diversity order using an STBC in the form of a matrix with orthogonal columns, as described with reference to FIGS. 1 and 2. However, because four complex symbols are transmitted for eight time intervals, the Tarokh STBC scheme brings a decrease by half in the data rate. In addition, since it takes eight time intervals to completely transmit one block with four complex symbols, reception performance is degraded due to channel changes within the block over a fast fading channel. In other words, the transmission of complex symbols through four or more Tx antennas requires 2N time intervals for N symbols, causing a longer latency and a decrease in the data rate.
To achieve a full rate in a MIMO system that transmits a complex signal through three or more Tx antennas, the Giannakis group presented a full-diversity, full-rate (FDFR) STBC for four Tx antennas using constellation rotation over a complex field. This FDFR STBC scheme will be described below.
FIG. 3 is a block diagram of a transmitter in a mobile communication system using the conventional Giannakis STBC scheme. As illustrated in FIG. 3, the transmitter includes a modulator 300, a pre-coder 302, a space-time mapper 304, and a plurality of Tx antennas 306, 308, 310 and 312. The modulator 300 modulates input information data (or coded data) in a predetermined modulation scheme such as BPSK, QPSK, QAM, PAM or PSK. The pre-coder 302 pre-codes Nt modulation symbols received from the modulator 300, d1, d2, d3, d4 such that signal rotation occurs in a signal space, and outputs the resulting Nt symbols. For simplicity, four Tx antennas are assumed. Let a sequence of four modulation symbols from the modulator 300 be denoted by d. The pre-coder 302 generates a complex vector r by computing the modulation symbol sequence, d using Equation (2):
                    r        =                              Θ            ⁢                                                  ⁢            d                    =                                                    [                                                                            1                                                                                      α                        0                                                  1                          .                                                                                                                                    α                        0                        2                                                                                                            α                        0                        3                                                                                                                        1                                                                                      α                        1                        1                                                                                                            α                        1                        2                                                                                                            α                        1                        3                                                                                                                        1                                                                                      α                        2                        1                                                                                                            α                        2                        2                                                                                                            α                        2                        3                                                                                                                        1                                                                                      α                        3                        1                                                                                                            α                        3                        2                                                                                                            α                        2                        3                                                                                            ]                            ⁡                              [                                                                                                    d                        1                                                                                                                                                d                        2                                                                                                                                                d                        3                                                                                                                                                d                        4                                                                                            ]                                      =                          [                                                                                          r                      1                                                                                                                                  r                      2                                                                                                                                  r                      3                                                                                                                                  r                      4                                                                                  ]                                                          (        2        )            where Θ denotes a pre-coding matrix. The Giannakis group uses a Vandermonde matrix, being a unitary one, as the pre-coding matrix. In the pre-coding matrix, ai is given as Equation (3):ai=exp(j2π(i+1/4)/4), i=0,1,2,3  (3)
The Giannakis STBC scheme uses four Tx antennas and is easily extended to more than four Tx antennas, as well. The space-time mapper 304 STBC-encodes the pre-coded symbols in the following method according to Equation (4):
                    S        =                  [                                                                      r                  1                                                            0                                            0                                            0                                                                    0                                                              r                  2                                                            0                                            0                                                                    0                                            0                                                              r                  3                                                            0                                                                    0                                            0                                            0                                                              r                  4                                                              ]                                    (        4        )            where S is a coding matrix for symbols transmitted through the four Tx antennas 306 to 312. The number of the columns of the coding matrix is equal to the number of the Tx antennas and the number of the rows corresponds to the time required to transmit the four symbols. That is, the four symbols are transmitted through the four Tx antennas for the four time intervals.
Specifically, for a first time interval, r1 is transmitted through the first Tx antenna 306, with no signals through the other Tx antennas 308, 310 and 312. For a second time interval, r2 is transmitted through the second Tx antenna 308, with no signals through the other Tx antennas 306, 310 and 312. For a third time interval, r3 is transmitted through the third Tx antenna 310, with no signals through the other Tx antennas 306, 308, and 312. For a fourth time interval, r4 is transmitted through the fourth Tx antenna 310, with no signals through the other Tx antennas 306, 308 and 310. Upon receipt of the four symbols on a radio channel for the four time intervals, a receiver (not shown) recovers the modulation symbol sequence, d, by maximum likelihood (ML) decoding.
Taejin Jung and Kyungwhoon Cheun proposed a pre-coder and concatenated code with an excellent coding gain in 2003, compared to the Giannakis STBC. Jung and Cheun enhance the coding gain by concatenating Alamouti STBCs instead of using a diagonal matrix proposed by the Giannakis group. For convenience sake, their STBC is called “Alamouti FDFR STBC”.
The Alamouti FDFR STBC will be described below. FIG. 4 is a block diagram of a transmitter in a mobile communication system using the conventional Alamouti FDFR STBC for four Tx antennas. As illustrated in FIG. 4, the transmitter includes a pre-coder 400, a mapper 402, a delay 404, two Alamouti coders 406 and 408, and four Tx antennas 410, 412, 414 and 416.
Referring to FIG. 4, the pre-coder 400 pre-codes four input modulation symbols, d1, d2, d3, d4 such that signal rotation occurs in a signal space. For the input of a sequence of the four modulation symbols, d, the pre-coder 400 generates a complex vector, r, according to Equation (5):
                    r        =                              Θ            ⁢                                                  ⁢            d                    =                                                    [                                                                            1                                                                                      α                        0                        1                                                                                                            α                        0                        2                                                                                                            α                        0                        3                                                                                                                        1                                                                                      α                        1                        1                                                                                                            α                        1                        2                                                                                                            α                        1                        3                                                                                                                        1                                                                                      α                        2                        1                                                                                                            α                        2                        2                                                                                                            α                        2                        3                                                                                                                        1                                                                                      α                        3                        1                                                                                                            α                        3                        2                                                                                                            α                        2                        3                                                                                            ]                            ⁡                              [                                                                                                    d                        1                                                                                                                                                d                        2                                                                                                                                                d                        3                                                                                                                                                d                        4                                                                                            ]                                      =                          [                                                                                          r                      1                                                                                                                                  r                      2                                                                                                                                  r                      3                                                                                                                                  r                      4                                                                                  ]                                                          (        5        )            where ai=exp(j2π(i+1/4)/4), i=0,1,2,3.The mapper 402 groups the four pre-coded symbols by twos and outputs two vectors each including two elements, [r1, r2]T and [r3, r4]T to the Alamouti coder 406 and the delay 404, respectively. The delay 404 delays the second vector [r3, r4]T for one time interval. Thus, the first vector [r1, r2]T is provided to the Alamouti coder 406 in a first time interval and the second vector [r3, r4]T is provided to the Alamouti coder 408 in a second time interval. The Alamouti coder refers to a coder that operates in the Alamouti STBC scheme .
The Alamouti coder 406 encodes [r1, r2]T so that it is transmitted through the first and second Tx antennas 410 and 412 for the first and second time intervals. The Alamouti coder 408 encodes [r3, r4]T so that it is transmitted through the third and fourth Tx antennas 414 and 416 for the third and fourth time intervals. A coding matrix used to transmit the four symbols from the mapper 402 through the multiple antennas is shown in Equation (6):
                    S        =                  [                                                                      r                  1                                                                              r                  2                                                            0                                            0                                                                                      -                                      r                    2                    *                                                                                                r                  1                  *                                                            0                                            0                                                                    0                                            0                                                              r                  3                                                                              r                  4                                                                                    0                                            0                                                              -                                      r                    4                    *                                                                                                r                  3                  *                                                              ]                                    (        6        )            
Unlike the coding matrix illustrated in Equation (4), the above coding matrix is designed to be an Alamouti STBC rather than a diagonal matrix. The use of the Alamouti STBC scheme increases a coding gain.
In the matrix S, an ith row denotes transmission in an ith time interval and a jth column denotes transmission through a jth Tx antenna. Specifically, r1 and r2 are transmitted through the first and second Tx antennas 410 and 412, respectively for a first time interval. −r*2 and r*1 are transmitted through the first and second Tx antennas 410 and 412, respectively for a second time interval. For a third time interval, r3 and r4 are transmitted through the third and fourth Tx antennas 414 and 416, respectively. For a fourth time interval, −r*4 and r*3 are transmitted through the third and fourth Tx antennas 414 and 416, respectively.
This Alamouti FDFR STBC, however, has the distinctive shortcoming of increased coding complexity because the transmitter needs to perform computations between all elements of the pre-coding matrix and an input vector, for pre-coding. For example, for four Tx antennas, since 0 is not included in the elements of the pre-coding matrix, computation must be carried out on 16 elements. Also, the receiver needs to perform maximum-likelihood (ML) decoding with a large volume of computation in order to decode the signal, d transmitted by the transmitter. While the FDFR STBC process has deficiencies, FDFR SFBC techniques are yet to be developed. Accordingly, a need exists for developing an FDFR SFBC technique with a minimal complexity and a minimal computation volume.
Orthogonal frequency division multiplexing (OFDM) is a promising technology to reduce channel fading in 4th generation (4G) mobile communication systems. Special consideration is being given to multi-user OFDM supporting multiple users in which each user is identified in the frequency domain. Since the implementation of an OFDM system involves consideration of channel changes in the frequency domain, space-frequency antenna diversity must also be exploited. That is, an SFBC scheme needs to be developed for the OFDM system.